Bulkhead Fitting Installation Guide | Apex Flow

A bulkhead fitting makes a sealed, threaded pass-through in a tank or vessel wall — the connection that lets you plumb a drain, fill, or feed line into a poly tank without it weeping around the hole. It seals against the tank wall with a gasket and a locknut, not with thread sealant, and almost every leaking bulkhead traces to one of three errors: the wrong hole size, the gasket on the wrong side, or an over-tightened nut that cupped the flange. This guide gives the hole-saw sizing chart, the correct gasket orientation, the install sequence, and a troubleshooting table.

Apex Flow Solutions stocks bulkhead fittings in polypropylene, PVC, and 316 stainless with EPDM, Viton (FKM), and Buna-N gaskets. The dimensions below are typical; confirm the required hole size against the specific fitting's data sheet before cutting — you cannot un-cut a hole.

Cutting into an expensive tank?

Hole size and gasket material are the two things you cannot fix after the fact. Send us your tank wall thickness, media, and the line size you're connecting and our team will confirm the bulkhead, gasket, and hole-saw size before you drill.

Anatomy of a Bulkhead Fitting

A bulkhead fitting has four parts: a body with a wide sealing flange on one end and a long threaded barrel on the other; a gasket (flat washer or O-ring) that seats under the flange; a locknut (backnut) that threads onto the barrel; and the threaded port (NPT or slip) on each end for the pipe connections. The flange and gasket sit against one face of the tank wall; the barrel passes through the drilled hole; the locknut draws up on the opposite face and compresses the gasket against the wall to seal. The connection threads (NPT) on the ends seal the pipes — but the wall seal itself is the gasket, not thread tape. The bulkhead's nominal "size" refers to the pipe connection (1/2", 3/4", etc.); the body barrel OD is larger and sets the hole size.

Hole-Saw Sizing Chart

Drill the hole to fit the threaded barrel with minimal clearance — too tight and the barrel won't pass, too loose and the gasket can't bridge the gap. The flange OD column tells you the minimum flat sealing area the tank wall must provide around the hole.

Bulkhead size (pipe conn.)	Required hole diameter	Approx. flange OD	Min. flat sealing area (dia.)
1/2"	1-1/4"	~2.0"	~2.25"
3/4"	1-1/2"	~2.4"	~2.75"
1"	1-3/4"	~2.9"	~3.25"
1-1/4"	2-1/8"	~3.4"	~3.75"
1-1/2"	2-3/8"	~3.9"	~4.25"
2"	2-7/8"	~4.7"	~5.25"

These are representative; flange and barrel dimensions vary by manufacturer, so always cut to the data-sheet hole size for your specific fitting. Use a sharp hole saw at low speed for plastic tanks to avoid melting or cracking, and deburr both faces.

Exploded view of a bulkhead fitting showing body, gasket, tank wall, and locknut in assembly order

Assembly order from the wet side out: fitting body and flange, gasket, tank wall, then locknut. The gasket seats between the flange and the tank wall — never under the locknut.

Which Side Does the Gasket Go?

This is the single most common mistake. The gasket goes on the flange side — between the wide flange and the tank wall — which is normally the wet side, inside the tank. The locknut threads onto the barrel on the opposite (usually dry/outside) face and pulls the flange tight, squeezing the gasket against the wall. Putting the gasket under the locknut instead leaves the flange sealing directly metal-to-plastic against the wall, which leaks. With a standard single-gasket bulkhead, install the body so the flange and gasket face the fluid; the head pressure then helps seat the gasket rather than push past it. Some heavy-duty bulkheads include a second gasket for the locknut side — follow the data sheet, but the primary seal is always the flange gasket.

Step-by-Step Installation

Mark the location on a flat area of the tank wall with the minimum flat sealing diameter clear of any seam, ridge, or curve. Drill the hole to the data-sheet diameter with a sharp hole saw at low speed, then deburr both faces smooth. Slide the gasket onto the barrel up against the flange. Insert the barrel through the hole from the gasket/flange side. From the opposite face, thread the locknut on by hand until snug, keeping the body from rotating. Tighten the locknut hand-tight, then about 1/4 to 1/2 turn more with a strap wrench or spanner — just until the gasket visibly compresses and the flange seats flat. Do not crank it; over-tightening cups the flange or cracks the locknut and actually breaks the seal. Apply thread sealant only to the NPT pipe connections, never to the wall seal. Fill and pressure-check before putting the system into service.

Gasket Material Selection

The gasket sets the chemical and temperature limit of the seal. Match it to the media just as you would a valve seat.

Gasket material	Best for	Avoid	Temp range
EPDM	Water, acids, bases, alcohols	Petroleum oils, fuels	-40 to 250°F
Viton (FKM)	Oils, fuels, aggressive chemicals	Hot water/steam, ketones	-15 to 400°F
Buna-N (Nitrile)	Petroleum oils, fuels, air	Ozone, strong oxidizers	-30 to 250°F
PTFE / FEP-encapsulated	Near-universal chemical resistance	High clamp load (cold flow)	-100 to 400°F

Curved & Thin Walls

A standard flat-flange bulkhead needs a flat sealing surface. On a curved tank wall (small-diameter cylindrical tanks especially), the flange can only contact the gasket on two edges, leaving a gap that leaks. Solutions: mount only on the flat ends or a molded flat boss, use a tank with a flat-spot fitting pad, or specify a bulkhead with a curved/contoured flange matched to the tank radius. Wall thickness also matters — the barrel must be long enough to pass through the wall and still expose enough thread for the locknut. Very thin walls may need a backing washer or reinforcing ring to spread the clamp load and prevent the wall from dimpling. Always verify the barrel length exceeds the wall thickness by enough to fully engage the locknut.

Cross-section of an installed bulkhead fitting showing the compressed gasket sealing against the tank wall

A correctly installed bulkhead: gasket compressed between flange and wall on the wet side, locknut snug plus a quarter to half turn on the dry side. The pipe threads are sealed separately with tape or paste.

Troubleshooting Leaks

Bulkhead leaks split into wall-seal leaks and pipe-thread leaks. Diagnose with this table.

Problem	Likely cause	Fix
Weeps around the flange at the wall	Gasket on wrong side or pinched	Reinstall gasket on flange/wet side, seated flat
Leaks despite a tight locknut	Hole too large; gasket can't bridge gap	Move to correct hole size or use larger flange
Locknut cracked / flange cupped	Over-tightened	Replace; tighten hand-tight + 1/4–1/2 turn only
Leaks on a curved tank wall	Flange can't seat on curved surface	Mount on a flat area or use contoured flange
Drip from the pipe connection, not wall	NPT thread under-sealed	Tape/paste the pipe thread; 2–3 turns past HT
Gasket swells or degrades over time	Wrong elastomer for the media	Switch gasket material per compatibility table

Standards & References

Bulkhead pipe-connection threads follow ASME B1.20.1 (NPT) or are solvent-weld slip sockets per ASTM D2467 (Schedule 80 PVC). Body materials are typically PVC/CPVC per ASTM D1784, polypropylene, or 316 stainless per ASTM A276. Gasket elastomers are specified by ASTM D2000. For potable water tanks, confirm the body and gasket carry NSF/ANSI 61 certification. Always follow the manufacturer's published hole-saw size and locknut makeup — flange and barrel dimensions are not standardized across brands.

Frequently Asked Questions

Which side of the tank does the gasket go on?

The flange side — between the flange and the tank wall, normally the inside/wet side. The locknut goes on the opposite face. Never put the only gasket under the locknut.

What size hole do I drill for a bulkhead fitting?

Drill to the manufacturer's specified hole diameter, which fits the threaded barrel with minimal clearance — typically about 1-1/4" for a 1/2" bulkhead up to 2-7/8" for a 2". Always confirm on the data sheet before cutting.

How tight should the locknut be?

Hand-tight plus about a quarter to half turn — just until the gasket compresses and the flange seats. Over-tightening cups the flange or cracks the nut and breaks the seal.

Do I use thread tape on a bulkhead fitting?

Only on the NPT pipe connections, not on the wall seal. The gasket seals against the tank wall; tape there would actually prevent proper gasket contact.

Can I install a bulkhead on a curved tank wall?

Not with a standard flat flange — it will leak. Mount on a flat area or molded boss, or use a bulkhead with a flange contoured to the tank radius.

Why does my bulkhead leak even though it's tight?

Usually the hole is too large for the gasket to bridge, the gasket is on the wrong side, or it is over-tightened and the flange has cupped. Check the hole size and gasket orientation first.